06/07/2023

Dr. Ing. Michele Trevisson

16:00 Uhr

In river systems characterized by sediment supply-limited conditions such as ephemeral rivers or gravel sand transitions, sediments are observed to form longitudinal ridges. The lateral patterns of fine and coarse sediment stripes are the result of sediment sorting induced by the presence of secondary currents. While most experimental studies on artificial ridges or with uniform/weakly bimodal sediments have focussed on the feedback mechanism between secondary currents and ridges, the morphological evolution of the ridges  has not yet been analysed. The talk will characterize the development of fine-sediment ridges over a rough bed composed of immobile spheres in a laboratory flume. The experimental setup is composed of a camera installed above the flume to measure the bed morphodynamics and of a side-looking 2D PIV system for flow measurements both over the troughs and over the ridges.

The morphodynamic measurements show that ridges and troughs develop through non-uniform erosion. While in the troughs, a constant sediment level below the top of the spheres is reached within a few hours, the ridges (crests) erode slower than the troughs over a period of several days. The ridge morphology is initially characterized by an increase in height and side-slope and a decrease in width. This is explained to be the result of a self-enhancing mechanism related to the enhancement of the secondary currents. After the troughs reach a constant bed level, the ridge height and side-slope then decrease, whereas the width remains constant. It will be shown furthermore, that the ridge spacing tends to a characteristic wavelength of 1.3 H. Thus, the number of ridges can be deduced from the integer number of characteristic wavelengths fitting into the width of the channel.

As the height of the ridges decreases, the PIV measurements will reveal a reduction in the intensity of the secondary currents, confirming the feedback mechanism. A quadrant analysis shows a considerable difference in the turbulent structure over the ridges and the troughs. Over the ridges, the strongest sweep events are displaced upwards by the local upflow resulting in reduced bed shear stress conditions, while over the troughs the sweeps are pushed towards the bed enhancing the local bed shear stress.